1. Field of the Invention
The present invention relates generally to processes for driving image display devices, and more particularly to an improved system and method for dithering video data. Even more particularly, the present invention relates to a system and method for dithering video data to be displayed on a display including an array of individual pixel cells.
2. Description of the Background Art
In recent years the demand for flat panel image/video displays has drastically increased, mainly because the overall volume and weight is significantly less than that of traditional CRT (cathode ray tube) displays of equivalent screen area. In addition, flat panel display devices are used in other applications unsuitable for conventional CRTs, for example in high resolution video projection systems. Examples of flat panel displays used in video projection systems include, but are not limited to, liquid crystal on silicon (LCOS) and deformable mirror devices (DMDs).
Today digital displays (e.g., LCDs) are common. When driving digital LCDs, the pixel is driven in one of two states: an “on” state or an “off” state. During the “on” state a saturation voltage potential is applied across the liquid crystal layer which results in the maximum light output (i.e., a light pixel or “on”). Conversely, the “off” state is obtained by applying a threshold voltage potential across the liquid crystal layer which results in the minimum light output (i.e., a dark pixel or “off”). Thus, at any given instant in time, a pixel is either on or off.
Because a digital LCD pixel only has two states, on or off, PWM (pulse width modulation) techniques have been employed so that a single pixel can display what appears to be other intermediate intensities. PWM involves modulating a pixel back and forth between two different states at such a rate the human eye integrates the two intensities to perceive a single intensity. For example, to display what appears to be a single intensity of 10% maximum brightness the “off” state is asserted 90% of the time frame while the “on” state is asserted the other 10% of the time frame. Similarly, to display what appears to be a single intensity of 75% maximum brightness the “off” state is asserted 25% of the time frame while the “on” state is asserted the other 75% of the time frame.
In a similar fashion, a method commonly referred to as dithering is used to display intensities unobtainable by single frame PWM. As an example, a particular type of dithering called temporal dithering is used to display intensity levels that are between the intensity levels that are attainable by PWM. Temporal dithering works similarly to PWM, except that temporal dithering modulates the values attained by PWM. In other words, PWM intensities are attained by modulating 0% and 100% intensities between time slices of a single frame while temporal dithering intensities are attained by modulating these PWM intensities over several frames of data. For example, to display the intermediate pixel value 127.25 on a single pixel, the value 127 is obtained from PWM and displayed three out of every four frames while the value 128 (also obtained from PWM) is displayed once every four frames. As a result, a greater number of intensity levels than defined by the PWM scheme can be achieved.
One problem associated with temporal dithering is that the number of displayable intermediate intensities between the PWM intensities are limited to the number of frames over which the data is dithered. For example, if a cycle includes a series of 10 frames, the only attainable intermediate intensities are tenths. Likewise, if the cycle includes a series of 4 frames, the only attainable intermediate intensities are fourths. For example, if the cycle includes 4 frames, the displayable intermediate intensities between N and N+1 are 1.25N, 1.5N, and 1.75N, N being an arbitrary intensity value defined by the PWM scheme, and N+1 being the next intensity value defined by the PWM scheme. Note that cycle refers to the sequence of frames needed to display a particular intensities.
Another dithering method, commonly known as spatial dithering, involves combining the simultaneous output of a plurality of pixels to achieve intermediate intensity levels. For example, a group of four pixels will appear to have a uniform value of 127.75 if three pixels are illuminated with a value of 128 and the other pixel is illuminated with a value of 127. Similarly, a group of four pixels will appear to have a uniform intensity value of 127.5 if two pixels are illuminated with a value of 127 and the other two pixels are illuminated with a value of 128.
One problem commonly associated with Spatial Dithering is that image resolution is sacrificed for the increase in intensity resolution. This is because it takes multiple pixels to make a single intensity value, rather than just modulating a single pixel to render a single intensity as described for pure temporal dithering. As an example, if an LCD includes groups consisting of four adjacent pixels that render what appears to be a single intensity, the resolution of the entire display will be four times less than it would be if each individual pixel were responsible for a single intensity.
FIG. 1 is a block diagram showing a prior art display driver circuit 100, which is operative to dither video data into planarized display data. In this particular embodiment, display driver circuit 100 includes dithering logic 102, a CLUT (color look up table) 104, and a planarizer 106. Dithering logic 102 receives video data 108 and frame count data 110 from a video data source 112 and frame count source 114, respectively. Further, dithering logic 102 performs dithering operations (e.g., temporal dithering described above) that depend on video data 108 and frame count data 110. Dithering logic 102 then outputs dithered video data 116 that is then received by CLUT 104, where it is mapped or converted to display data 118. Planarizer 106 receives and converts display data 118 into planarized display data 120. A display 122 (e.g., LCD) then receives planarized display data 120 and displays a corresponding intensity.
One problem with prior art circuit 100 is that the number of displayable pixel values are limited by the size of the data word received by the dithering logic. For example, if display driver circuit 100 is driven by 8-bit data words, then only 256 different values can be defined, before modulation techniques are applied. So, the smallest increments between intensity values is limited to the value of data word's LSB (least significant bit). For example, if a dithering logic process adds a bit value to an 8-bit data word, the original value is increased by a value of 1/256 which is approximately 0.3906% of the maximum value.
Another problem is that the electro-optical response curve of the some displays (e.g., LCDs) is not linear. As a result, even if display data can be dithered to precisely achieve an intermediate root-mean-square (RMS) voltage, that RMS voltage may not produce the desired intensity output.
Other known methods for displaying intermediate intensity values involve estimation techniques. However, estimating values leads to noticeable image problems such as the appearance of “steps” or “lines” in contoured images. The appearance of such “steps” is a result of a an estimated intensity value being more different than it's true value than that of an adjacent intensity value being displayed on adjacent pixels.
What is needed, therefore, is a display driving circuit and method capable of more accurately displaying intensity values on a pixel or group of pixels. What is also needed is a display driving circuit and method that eliminates visual artifacts from displayed images.